Food procurement and processing

Fish display a wide range of adaptations of the mouth and pharynx for specific feeding patterns including planktivory, fin-biting, picking and scraping. Appetite control is complex, involving stimulatory and inhibitory hormones. The gut has a linear plan similar to other vertebrates but with considerable variation between taxa, and a stomach may be absent. Many bony fish possess pyloric caeca, containing digestive enzymes, and may increase surface area for digestion. In chondrichthyes (sharks, etc.), a ‘spiral valve’ increases surface area of the intestine. Smooth muscle contractions in the gut wall pass food along the tract under control of food pressure, the autonomic nervous system and specific peptides. Digestion by hydrolytic enzymes, and absorption occur in the intestine, monomers produced being absorbed mainly through transcellular routes, involving enterocytes, into the blood of the hepatic portal vein to the liver. Dietary requirements and nutrition are discussed.